The present invention relates to synchronous machines of the type comprising an ac winding and an inverter or like static switch connected to the ac winding.
Synchronous machines have been known, for example, as the noncommutator motor comprising an ac winding, a static switch connected to the ac winding, and a dc winding. In this type of synchronous machine, the current in the ac winding should be commutated. To this effect, a turn-off at the static switch is supposed to occur at a timing after decrease in the current in the static switch. In an electric valve used as the static switch a turn-off at the switch can take place only after decrease in the current in the switch. In a transistor switch, it is also desirable that its turn-off occurs at a timing at which the current in the switch is already on the decrease, in order to reduce power for turn-off switching. Further, in a synchronous machine in which on-off operation at the static switch causes the current in the ac winding to be sequentially changed, the prime aim of such machine is to bring about change (or commutation) in the current in the ac winding. In this specification, therefore, the recovery of nonconductivity (i.e., turn-off, or extinction in the electric valve) of the static switch is discriminated from the sequential change in the current in the ac winding; the sequential change in the current therein will hereinafter be referred to as commutation.
In a prior art synchronous machine control system, the separately excited commutation method has been employed in which the commutation is done by an electromotive force ascribed to magnetic fluxes from the ac winding and from the dc winding, the fluxes being in motion relative to each other. Another prior art system has employed a suitable means for use with the static switch. In practice, however, the former approach is undesirable in that commutation is hampered from occurring in the event of low relative velocity of flux motions. The latter is also impractical in that the construction of the static switch is intricate.